1. Field of the Invention
The present invention relates to noise suppression and isolation in electronic systems, and in particular, to maintaining stopband characteristics of an electromagnetic bandgap structure in a circuit board having multiple conductive layers.
2. Background of the Related Art
In modern high-speed and mixed-signal electronic systems, isolating power/ground noise coupling between circuits on a printed circuit board (“PCB”) is a challenge. If not controlled or accounted for, the noise coupling between circuits may result, for example, in false switching for digital circuits and malfunctioning of analog circuits.
One approach to suppressing noise coupling is to provide a split power/ground plane. However, the split power/ground plane requires the use of multiple DC power supplies, which increases cost. Another approach to suppressing noise coupling in electronic circuits is the use of periodically patterned structures, such as electromagnetic bandgap (“EBG”) structures. EBG structures exhibit stopband properties tending to prevent or reduce electromagnetic propagation in the range of stopband frequencies. Unlike the use of a split power/ground plane, EBG structures can be used with circuits sharing a common power supply.
In a conventional EBG structure, a periodically patterned structure is provided in one layer (e.g. in a power plane) of a PCB, and a solid metal layer is provided in another layer (e.g. in a ground plane). The solid metal layer is spaced from the patterned layer, typically by a single substrate layer. Together, the periodically patterned layer and the solid metal layer create a noise filter. However, options for including additional conductive layers in the PCB along with an EBG structure are conventionally limited, because the total electrical characteristics of the EBG structure would be changed by the presence of an additional solid metal layer above the patterned layer. In particular, the stopband characteristic of the EBG structure would be diminished similar to the manner in which a microstripline would be nullified by the presence of another ground plane above it.